Fibroblasts are the predominant cells and collagen is the most abundant extracellular component in the periodontal ligament (PDL). The relationship between fibroblast polarity and orientation and the genesis of the 3-D architecture of the extracellular matrix is of increasing interest to cell biologists. Furthermore, the interrelationship between cytostructure and function during collagen secretion and fibrillogenesis contains information of significance not only to cell biology but also to a better understanding of the biology of the PDL. Greater understanding of PDL fibroblasts is of general importance to research in periodontal health. We propose to conduct research into six areas of fibroblast activity: 1) Studies of the cytological and synthetic aspects of fibroblast differentiation in relation to the development of the PDL fiber bundles. 2) Ultrastructural immunolocalization of C- and N- propeptides of collagen in fibroblasts and fibrils so as to learn more about their role in fibrillogenesis. 3) Immunocytochemical studies of collagen types I, III, V and fibronectin during fibroblast differentiation and PDL development. 4)\Studies of the relationship between structure and function of the Golgiapparatus of PDL fibroblasts. 5) The localization of receptors for insulin, growth hormone and fibroblast growth factor on PDL fibroblast membranes during cytodifferentiation and PDL development. 6) Immunocytochemical localization of cathepsin D in periodontal ligament fibroblasts at the ultrastructural level.